Scatterometry is an optical technique which uses the optical properties of scattered light to determine physical attributes of a sample. More specifically, scatterometry is used in the field of semiconductor testing to monitor the line width and line shape of patterns on semiconductor wafers. This is accomplished by forming a test structure including an array of lines on the semiconductor wafer. Typically the test structure is a diffraction grating some tens of microns on a side, but other structures can be used. Light is directed to the test structure and the light reflected from the test structure is measured at different wavelengths and scattering angles. These measurements are recorded and provide an optical “signature” of the test structure. This signature is then compared to a library of signatures for known structures, in a technique referred to as “inverse scattering.” The library of known structures may be determined either by measurement or modeling. When the test data correlates to a signature of a known structure, the physical attributes of the test structure are assumed to be the similar to those of the corresponding known structure. As an example, a certain signature might indicate that the edges are rounded and that the line widths have a specific dimension. In this manner, the unknown physical attributes of a test structure are determined from the optical properties of the light scattered from it.
Scatterometry is becoming increasingly important as a method for accurately measuring the dimensions, shape and overlay registration of diffraction gratings on semiconductor devices. Scatterometry is used for measuring film thickness, measuring optical parameters of thin films, and measuring the dimensions of arrays of contact holes on semiconductor devices. Further still, scatterometry is used in metrology to provide control feedback in various manufacturing processes for integrated circuits, including both lithography and etching processes.
Current scatterometers are not capable of simultaneously measuring scattering data at multiple angles and wavelengths. In one type, a single illumination wavelength is provided by a laser beam and scattering data is generated one angle at a time by mechanically changing the angle of the illuminating laser beam while simultaneously moving the detector to receive the specularly reflected light. In another type, illumination is at a fixed angle (or a narrow cone of fixed angles) and the illumination is over a range of wavelengths. In another type, an ellipsometer is used to collect scatterometry data. In yet another type, broad-band illumination is used at a small cone angle of illumination angles, and the angle of incidence and the detector location are varied to acquire data for a multiple of angles.
Examples of the background and prior art include:
4,199,219April 1980Suzki et al.356/445
This is an early patent on the idea of optically scanning a light beam across a pattern on a wafer and detecting the reflected intensities for the purpose of measuring geometrical parameters of a pattern.
4,583,858April 1986Lebling et al.356/446
In this patent a method is described for illuminating an object with beams oriented at a number of different angles relative to the normal and with the ability to switch some of the channels off and on in order to avoid confusion in the final detector.
4,710,642December 1987McNeil250/571
A laser beam illuminates the object at a single angle of incidence, and scattered radiation is detected at a multitude of angles.
4,806,018February 1989Falk356/446
In this patent, light from a single point in the back focal plane of a lens illuminates an object with a collimated beam at an angle relative to normal. Different detectors are used to detect the scattered radiation at a plurality of angles.
5,164,790November 1992McNeil et al.356/445
A laser beam illuminates a diffraction grating object at a fixed angle, and diffracted light is detected. Results are compared with simulated diffraction models to provide calibration.
5,241,369August 1993McNeil et al.356/445
This patent shows a technique for measuring the non-specular scattering off of an object which is illuminated by a collimated beam.
5,539,571July 1996Cabib et al.392/019
A spectral decomposition of each pixel of an object is created by means of Fourier transform spectroscopy. Does not use a two-beam interference microscope. Rather, the light emitted from an object is split into two beams downstream in the output channel and each pixel is analyzed in a Fourier Transform spectroscopic system to determine the spectral emission of each pixel separately. In this system the interference beam splitting occurs after the light has scattered off of the object. In an interference microscope the interference beam splitting occurs before the light hits the object.
5,633,714May 1997Nyyssonen359/225
This patent describes an interference microscope system having a narrow angle, single wavelength, polarized illumination. The Fourier transforms mentioned in this patent are spatial Fourier transforms performed in the image space and not path length Fourier transforms as used in Fourier spectroscopy. The complex amplitudes (i.e. phase and amplitudes) of the scattered fields are measured in the vicinity of the focus plane of the microscope system, and not at the back focal plane. In this system, the object itself is imaged and not the back focal plane of the objective lens. This technology cannot eliminate the confusion regarding which illumination angle caused scattering into which output angle, and therefore it cannot be used to measure the specular reflectivity amplitudes for a non-specular object like a diffraction grating without limiting in some way the illumination angles.
5,703,692December 1997McNeil et al.356/445
An opto-mechanical means for changing the angle of incidence of a collimated beam on an object as a function of time. A single angle of incidence is measured at one time.
5,856,871January 1999Cabib et al.356/346
This patent uses spectral imaging of the type shown in U.S. Pat. No. 5,539,571 for film thickness measurement. Since for uniform films there is no non-specular scattering, there is no confusion regarding which output angle came from which input angle.
5,867,276February 1999McNeil et al.356/445
In this patent, the angle of incidence is changed by activating a mechanical stage to rotate the object, and the reflected specular light is analyzed in a spectrometer.
5,912,741June 1999Carter et al.356/445
This patent shows a novel beam steering system for illuminating the object at one angle at a time and collecting both diffuse and specular reflected light.
5,963,329October 1999Conrad et al.356/372
This is a broad scatterometry patent utilizing reflectometry, but it does not include an interference microscope or provide for measurement of interference images of a back focal plane from which can be deduced the specular reflection amplitudes that are shown in the current patent.
5,923,423July 1999Sawatari et al.356/484
This is a defect finding system which utilizes a two beam interference design, but is not a Mirau or Likik configuration. Rather, this system is teaches the use of oblique angles of illumination. Particles are detected by measuring interference between forward scattering and back scattering. The Doppler shift of a moving particle on a wafer is used as a phase shifting mechanism for observing the interference.
6,429,943August 2002Opsal et al.356/625
This patent shows illumination over a multitude of angles and also a multitude of wavelengths. However, it does not contain an interference microscope. Only reflected intensities can be measured and not complex amplitudes, and the illumination aperture must be restricted if confusion is to be avoided between specular and non-specular reflections in some cases.
Two general references on Fourier spectroscopy are: Vanasse, G. A., Sakai, H., “Fourier Spectroscopy,” in Progress in Optics Volume VI, Edited by E. Wolf, North-Holland Publishing, Amsterdam, 1967, pages 259–330, and Steel, W. H., Interferometry, Cambridge University Press, London and New York, Second edition, 1983.
It is desirable to collect scattering data for a wide range of wavelengths and over a range of scattering angles. It is further desirable to do this quickly with an apparatus that has a minimum number of moving parts. Further still, it is desirable to illuminate the sample at multiple angles and wavelengths simultaneously in order to provide for a faster test. However, if the sample is illuminated at many angles at once, it is difficult to determine the intensity of the specular radiation measured by a detector. This is because light scatters in both specular and non-specular modes. That is, light incident from a given angle scatters in two modes. First, it scatters in a specular mode, where the angle of reflection equals the angle of incidence. Second, it scatters in a non-specular mode, where the angle of reflection is different from the angle of incidence. Thus, light received by a detector may include both light scattered in the specular mode from one incident angle and light scattered in a non-specular mode from a different incident angle. However, it is necessary to distinguish the specular terms from the non-specular terms in the data in order to develop an accurate “signature” for the sample. Since it is difficult to distinguish light scattered in a specular mode from light scattered in a non-specular mode, spectrometers typically illuminate a sample at only one incident angle (or a narrow range of incident angles) at a time. If the detector is then positioned to receive only light reflected at this one angle, only light reflected in a specular mode would be detected. However, this adds time to the detection process and complexity to spectrographic instrument since it must include a mechanism for varying the angle of the illumination and/or detection if multiple angles are to be measured.
It is therefore an object of the invention to provide a scatterometer system that enables multiple illumination angles to be measured simultaneously while obtaining amplitude data for each angle of illumination and for a range of wavelengths.
It is a further an object of the invention to measure scattering data at multiple angles and multiple wavelengths simultaneously without the need to reposition the optical components or the object.
It is a further object of the invention to measure phase and polarization dependence of the specular scattering data.